1. Field of the Invention
The embodiments disclosed herein relate to switching power supply apparatuses, and in particular relate to a switching power supply apparatus which includes a current-resonance type DC-DC (direct current-direct current) switching converter and which may handle burst control and the like and detect a load state with high precision.
2. Background of the Related Art
The current-resonance type DC-DC switching converter is widely employed as the power supply adapter of a television or the like because it is suitable for achieving an increase in efficiency and a reduction in thickness. In a switching power supply apparatus connected to an AC power supply for a load exceeding a predetermined level, a DC voltage kept constant by a power factor correction (PFC) circuit is used as its input voltage.
Such a switching power supply apparatus detects a load state indicating whether a load is heavy or light, in addition to controlling an output voltage to be constant, and when the load is light, performs, for example, a burst operation of temporarily stopping switching. This load state may be known by directly detecting a current output from the secondary side of a transformer for current resonance. However, because the detected load state needs to be transmitted to a control IC (Integrated Circuit) on the primary side of the transformer, the cost of a feedback circuit therefor poses a problem. For this reason, a load state is usually detected on the primary side of a transformer.
In detecting a load state on the primary side of a transformer, there is known a method for detecting, as a load state, the current flowing through a resonant circuit (e.g., see Japanese Laid-open Patent Publication No. 2012-170218, paragraphs [0050] to [0052] and FIG. 4). According to the description of Japanese Laid-open Patent Publication No. 2012-170218, the load component of a current flowing through the resonant circuit is detected in synchronization with an on-period of a high-side switching element or a low-side switching element. Accordingly, the detectable current is the load component only during a period when a high-side or low-side switching element is turned on, i.e., a one-sided load component during switching. This method is practical only on the premise that a state in a period in which the other switching element is turned on and current detection is not performed is identical to a state in a period in which current detection is performed. Accordingly, the high-side or low-side switching element needs to continue to be operated always with the same on-time ratio (i.e., 50%) (balance-controlled). Consequently, this is not a current detection method accurately indicating a load state. In the following, the burst operation and unbalanced operation will be described as an example unable to accurately indicate a load state.
First, a case is described where the burst operation is performed. Consider a current resonance converter, in which high-side and low-side switching elements are alternately controlled with the same on-width. Here, in performing the burst control with a 10% duty ratio (e.g., with the switching operation period of 1 millisecond (ms) and the switching stop period of 9 ms at a switching cycle of 10 microseconds (μs) (the switching frequency is 100 kHz)), the actual load is only 10 watt (W) even if the load component during the switching operation period is recognized as 100 W. This is expressed with the formula belowPload=Pdet×D where Pload is a load, Pdet is the load component during the switching operation period, and D is the duty ratio of burst. Therefore, even if the load component during the switching operation period is detected, a load state will not be able to be detected unless the burst duty ratio is known.
Next, an operation state (the so-called unbalanced control) in which the high-side and low-side switching elements are alternately controlled with different on-widths (e.g., see Japanese Laid-open Patent Publication No. 2006-204044, paragraph [0020]) will be described. Here, assume that the high-side switching element operates with the on-width of 30% and the low-side switching element operates with the on-width of 70%, for example. In this case, if a load component is detected during the high-side operation period in accordance with the scheme of Japanese Laid-open Patent Publication No. 2012-170218, the load state will be recognized as an almost no-load state. On the other hand, if a load component is detected during the low-side operation period, the load will be detected larger than the actual load. As described above, because a load component has the dependency on the high-side and low-side operation rates, a load state is not accurately detected.
As described above, in the technique of Japanese Laid-open Patent Publication No. 2012-170218, a load state is estimated by detecting the current flowing through a resonant circuit in synchronization with the on-period of the high-side switching element or low-side switching element, and therefore there is a problem that the technique is unable to handle the burst control or the like.